Temperate grasslands and forests

Temperate forests can swing from −30 °C in winter to 30 °C in summer, and their trees drop every leaf before the cold arrives.

Prairies and steppes

Temperate grasslands cover much of central North America, where they are called prairies, and parts of Eurasia, where they are called steppes. Summers are hot and winters are cold. Plants can grow only when it is warm enough and water is available, which means spring, summer and autumn; in winter the water is frozen and growth stops. Annual precipitation is 25 cm to 75 cm, too little for most trees except along rivers and streams.

What grows instead is dense grass, thick enough to feed large herds of grazing animals such as the American bison, which once lived on the prairies in huge numbers. The soil beneath is packed with roots and rhizomes, the underground stems of the grasses. They anchor the plants and, when they die and decay, add organic matter to the soil. That is why grassland soils are among the most fertile on Earth and why so many of them are now farmland.

Fire, mostly started by lightning, is a natural part of grasslands. OpenStax explains that when fires are stopped for a long time, the grassland slowly turns to scrub and sometimes dense forest of drought-tolerant trees. Restoring a prairie often means setting controlled burns on purpose to keep the trees back.

Temperate forests and falling leaves

Temperate forests are the most common biome in eastern North America, western Europe, eastern Asia, Chile and New Zealand. Temperatures range from −30 °C to 30 °C, with freezing spells in winter, and precipitation of 75 cm to 150 cm is spread fairly evenly through the year. The dominant plants are deciduous trees, which drop their leaves each autumn and stay bare through the winter. With no leaves there is no photosynthesis, so the trees rest. In spring new leaves open as the air warms.

That winter rest costs the forest some growth: the net primary productivity of temperate forests is lower than that of tropical wet forests, and they hold fewer tree species. They are also more open, because most of their trees do not grow as tall. Their great strength is the soil. Each autumn a thick layer of leaf litter covers the ground. As it decays it returns nutrients, protects the soil from erosion, insulates the ground and gives a home to small animals such as pill bugs and their predators, such as the red-backed salamander. Tropical rainforests do not build up such a layer.

Where grassland meets forest

Look at the two rainfall ranges side by side: grasslands top out at about 75 cm a year, and temperate forests start at about 75 cm. Around that line the two biomes trade places with small changes in rain, soil and fire. Trees already line the rivers that cross the prairie, and OpenStax notes that where fire is kept out for long, trees and shrubs move into the grass; controlled burns push them back. Both biomes have the same seasons, with growth in spring, summer and early autumn and a pause in winter, so here it is mostly water, not temperature, that decides which one you see.